Typically nuclear fission reactors for power generation are housed within a containment structure as a safety measure. Nuclear reactor containments are designed and employed to enclose the nuclear reactor pressure vessel containing the core of heat generating fissionable fuel and ancillary components of the system, such as portions of the coolant/heat transferring fluid carrying conduits or other associated means which constitute a source of and/or means for the conveyance of radiation and/or fission products. As such, the containment structure housing a nuclear reactor plant must effectively isolate the reactor and related components enclosed within its confines by sealing-in all contents including any water, steam, gases or vapor and entrained fission products or other sources of radiation that may have escaped from the reactor pressure vessel and in particular from its associated cooling and heat transferring system including the main steam condensed coolant water loop.
In the event of certain malfunctions in a nuclear reactor system, such as a significant loss of coolant accident which may be due to a major breach of a main conduit, large volumes of very hot pressurized water and/or steam may be released into the interior of the containment structure. The very hot pressurized water, which is likely to carry along entrained radioactive fission products, will flash into steam and in turn substantially increase the pressure and temperature within the containment structure. Accidents of this nature can result in very high pressure and temperature conditions within the confines of the leak proof containment structure whereby such an occurrence has the potential of impairing the integrity and/or function of the containment structure.
Potentially high pressures and temperatures due to the inherent high thermal energy produced by a reactor and flashing steam cannot be released by venting from the containment structure or otherwise be permitted to escape into the outside atmosphere since the steam vapor and fluid may entrain and carry radioactive fission products which would also be released into the environment.
Common commercial boiling water cooled and moderated nuclear fission reactors employed for electrical power generation are normally provided with one or more suppression pool chambers within the reactor plant containment structure. A suppression pool chamber is utilized to provide a large reservoir of cooling water available for condensing steam released from the reactor pressure vessel and its associated steam/coolant water loops or circuits, or escaping therefrom due to an accident, and also for further cooling down condensed steam and released or escaped hot water. Suppression pool chambers commonly are circular or semicircular with one or more pools extending substantially around the fuel containing reactor pressure vessel at an appropriate elevation, and may be constructed as a single chamber or several individual chambers each containing a condensing and cooling pool of water. Appropriate fluid conduits are provided for conveying steam and/or hot water to below the surface of the water of the pool in the suppression chamber for condensing the steam and/or cooling hot water.
Recent proposals in the nuclear reactor industry comprise various passive or self-activating or operating heat removing safety systems that function by means of inherent natural phenomena such as a hydrostatic head, pressure differences and/or fluid heat convection which provide added safety measure. One such system comprises the inclusion of an isolation condenser system comprising one or more closed vessels holding a pool of cooling water which are housed within the reactor containment structure and sealed to isolate the pool from the atmosphere of the containment structure and any possible containments therein. Such an isolation container(s) with its isolated cooling water is vented to the external atmosphere to release any significant increases in pressure or heat introduced through its cooling function. Thus, excessive heat can be disposed of by indirectly transferring it to the cooling water of an isolation condenser and in turn dissipating it out into the outside atmosphere without permitting the escape of any radioactive contaminants out into the atmosphere.
One or more closed heat exchangers are provided submerged within the pool(s) of cooling water contained within the isolation condenser vessel(s) for transferring heat energy to the cooling water, which in turn can be vented out into the external atmosphere. Such heat exchangers provide for passing steam, hot water and gases from the reactor pressure vessel and its related heat conveying circuits, and/or from the containment structure atmosphere through closed fluid carrying ducts immersed in the isolation condenser cooling pool(s) for dissipation of heat into the isolated cooling water. Excessive heat can thus be dispelled indirectly out into the atmosphere free of any radioactive contaminants while returning cooled fluid back into the reactor system with any entrained contaminants. Liquid water condensate produced from steam cooled in a heat exchanger immersed in the cooling pool at an isolation condenser can be conveyed back into the reactor pressure vessel or related components for use as a coolant.
Additionally these passive safety heat removing systems can comprise one or more elevated, gravity feed auxiliary coolant water supply pools retained in a chamber(s) or vessel(s) located within the containment structure at an elevation substantially above the core of fissionable heat producing fuel within the reactor pressure vessel. This gravity feed auxiliary coolant supply pool is available to provide additional cooling water to the fuel core and/or other components to preclude overheating due to an accidental loss of the reactor's operating coolant.
One recently proposed arrangement of a composite passive cooling means for boiling water cooled nuclear reactor plants comprises a unique system for interconnecting several components provided for condensing steam and cooling aspects of such reactor plants to enhance safety and performance. This interconnecting arrangement comprises adding at least one fluid carrying conduit having a valve for optionally controlling fluid communication between the interior of the reactor pressure vessel and the inlet into one or more heat exchanger units submerged in the isolated pool of cooling water of the isolation condenser(s). Thus, on the occurrence of a mishap or reactor shutdown, excess heat due to the malfunction, or of decay of the nonfissioning fuel during shutdown, can be transferred to a heat exchanger unit within an isolation condenser for dissipation. The control valve is provided with manual or automatic means for its operation upon the occurrence of an event calling for additional cooling capacity.
Additionally the interconnecting arrangement comprises a fluid carrying conduit(s) similarly leading into the inlet of one or more heat exchanger units of the isolation condenser(s) and having a valve for optionally controlling its reception of fluid from the area external of the reactor pressure vessel and within the containment structure. Thus, upon steam and or hot water escaping or being released out from the reactor pressure vessel or associated conduits, which raises the temperature and/or pressure within the containment housing structure, fluid of the overheated and/or pressurized atmosphere within the containment can be conveyed through the valve controlled fluid carrying conduit into the inlet of the heat exchanger unit(s) for condensing and/or cooling. This releases any excessive temperature and/or pressure occurring within the containment housing structure thereby precluding possible damage to the isolated containment structure.
A further aspect of this proposed arrangement comprises providing a fluid communicating conduit extending down from the outlet(s) of the heat exchanger unit(s) submerged in the cooling water pool of the isolation condenser(s) to the elevated gravity feed auxiliary coolant water supply pool(s). This conduit conveys liquid condensate from the heat exchanger unit(s) to the gravity feed auxiliary coolant water supply pool(s) for the purpose of augmenting or resupplying cooling water thereto and in turn reuse as a cooling medium.
To inhibit any rapid combining reaction of oxygen from the containment atmosphere with hydrogen released by a reaction of steam components of the reactor fuel core such as fuel element zirconium container metal or alloys, which reaction can result in a generation of hazardous hydrogen following a mishap that has caused the fuel materials to reach abnormally high temperatures, the area within the containment structure housing may be purged of atmospheric air and filled with nitrogen gas. However, nitrogen, which is noncondensible under reactor operating conditions, can be forced by high pressures produced within the containment housing structure along with other fluids such as steam into the conduit carrying fluids through the heat exchanger unit(s) submerged in the cooling water pool of the isolation condenser(s). Noncondensible gases such as nitrogen, carried along mixed with liquid water condensed from steam within the heat exchanger unit(s), cannot be utilized as a reactor coolant and returned into the reactor pressure vessel along with condensate water, or in related coolant systems. To deal with this possible situation, a liquid/vapor phase separating device is provided to segregate condensed liquid from such a mixture as a liquid condensate component for recycling and coolant service within the reactor system, and isolates the vapor component comprising noncondensible gas such as nitrogen together with any remaining uncondensed steam and other gases. This latter vapor component is conveyed to the suppression pool for further cooling and condensation of any remaining uncondensed steam. To accomplish this final cooling and condensation process one or more fluid carrying conduits is provided extending from a vapor outlet of the liquid/vapor phase separator to just below the surface of the water pool in the suppression pool(s) to attain further cooling and condensing of the separated vapor.
The term "maximum submerged depth" is used to describe the range of acceptable elevations of the outlet (56) of conduit (54) communicating with the liquid/vapor phase separating component projecting down beneath the surface of the water pool (24) in the suppression pool chamber 22.
Referring to the diagram shown in FIG. 2 of the drawing, the term "maximum submerged depth" is defined by the following. S/P illustrates the normal operating level of the water pool in the suppression pool chamber, and D/P the drawdown level. R/P illustrates the range of acceptable elevations (submergences) of the outlet of the conduit (C) communicating with the liquid/vapor phase separating component projecting down beneath the water pool surface of the suppression pool chamber. Zone A illustrates a zone of elevations in the water pool of the suppressions pool for which, if the conduit is terminated anywhere within this zone, desired steam/vapor flows through the heat exchanger and then through/along conduit (C) and terminating within the suppression pool are no longer the favored pathway (as governed by circuit pressure-drop process) compared with alternative pathways into the suppression pool (provided by the containment design configuration) for this steam/vapor proclivity to find its way into the lowest pressure region of the containment.
The outlet of conduit C is positioned within a narrow range of acceptable elevations within the suppression pool. By so positioning the outlet of the conduit within this range, a preferred flow pathway is afforded for steam vapor produced in the reactor. Along this "preferred pathway" is the heat exchanger followed by the liquid/vapor phase separator followed by the said conduit so that by the fluid media thus flowing along this preferred pathway a desired heat rejection (to the external pool) consequential to steam condensation will occur in a continuously-occurring mode. The pool elevation coinciding with the uppermost boundary to the said range of acceptable elevations is that particular elevation corresponding to the drawdown elevation to which the suppression pool depth contracts, following an accident or event for which the subject cooling system is designed to mitigate. The pool elevation coinciding with the lowermost boundary to the said range of acceptable elevations corresponds to the "maximum submergence depth" for the conduit, below which the desired flow pathway through/along the said conduit no longer remains the favored pathway for steam vapor to communicate with the suppression pool, because of the presence of alternative flow pathways presenting lower pathways flow resistance for other purpose by the pressure suppression containment design.